FIG. 1 illustrates a block diagram of an exemplary prior art uninterruptable power supply (UPS) 100. The UPS 100 includes an AC-DC rectifier 110 which converts AC line voltage 105 to DC 115. DC 115 is relatively high voltage. The UPS 100 further includes a DC-AC inverter 120 which converts the DC 115 to the AC output 125. The UPS 100 further includes a bidirectional DC-DC converter 130 and a battery 140.
When the AC line voltage 105 is normal, the bidirectional DC-DC converter 130 operates in buck mode, meaning that it reduces (bucks) the relatively high voltage DC 115 down to the lower voltage DC 135. The bidirectional DC-DC converter 130 outputs DC 135, which charges the battery 140. The voltage of DC 135 is chosen based on the battery 140.
When the AC line voltage 105 fails, the bidirectional DC-DC converter 130 operates in boost mode, meaning that it increases (boosts) the relatively lower voltage DC 135 up to the higher voltage DC 115. The battery 140 feeds the bidirectional DC-DC converter 130 that, in turn, feeds the DC-AC inverter 120, which feeds the load.
In some applications, deploying multiple UPS sharing a common battery may be desirable. In such applications, two or more bidirectional DC-DC converters, such as converter 130, would be connected in parallel to one battery or one battery string that is common to the multiple UPS.
FIG. 2 illustrates a schematic of an exemplary prior art bidirectional DC-DC converter 200.
The converter 200 includes a first set of capacitors C1 and C2 connected to a positive rail and a negative rail, respectively, of a DC Link. The converter 200 further includes buck switches Q1 and Q2, boost switches Q3 and Q4 and diodes D1, D2, D3, and D4. The buck switches Q1 and Q2 switch on and off periodically in buck mode and remain off in boost mode. The boost switches Q3 and Q4 switch on and off periodically in boost mode and remain off in buck mode. The converter 200 also includes inductors L1 and L2, and current sensors CS1 and CS2. The converter 200 also includes a second set of capacitors C3 and C4 connected to the battery BAT.
The converter 200 may be connected in parallel with other bidirectional DC-DC converters (not shown) to a common battery BAT. The converter 200 is shown connected to the battery BAT through three terminals, +BAT, −BAT, and midBAT. In this configuration, the battery BAT must have a midpoint connection point midBAT, which may be a disadvantage because it adds additional connections and cabling. The converter 200 may also be connected (not shown) to the battery BAT through only two terminals +BAT and −BAT with no midpoint connection. However, this configuration has disadvantages relating to voltage centering of the DC Link relative to the common battery BAT and circulating current between parallel connected converters connected to the common battery BAT. Additional control circuitry for active centering or active balancing is usually needed to mitigate these disadvantages with prior art converters. The additional circuitry adds cost and complexity.